This invention relates to an apparatus for electrically discharging a shaft and more particularly a rotating shaft of an AC or DC motor employing a solid-state, variable-speed drive.
It is known that bearing assemblies suffer premature wear, i.e., "fluting", when journalling shafts carrying excessive voltage levels. A shaft of an electric motor, or even a non-driven roller, can inductively or capacitively couple electrical energy such that voltage or current levels may exceed a given threshold, thereby causing an electrical discharge through a bearing/race discharge site. The discharge pits the bearing and race at the vicinity of the discharge and the pitting damage results in fluting of the bearing race assembly. Electric motors are especially vulnerable to such bearing degradation when employing solid-state, variable-speed drives.
Isidor Kerszenbaum, "Shaft Currents and Electrical Machines Fed by Solid-State Drives", 1992 IEEE industrial and Commercial Power Systems Technical Conference, 1992 Conference Record, pages 71-79, provides two primary theories according to which solid-state switching vices of a variable-speed drive induce current in a motor shaft. According to the firs current inducement method, "dissymmetry effects", the reluctance path of a motor changes as a function of the angular position of the motor's rotor due to magnetic asymmetries of either a stator or a rotor core. For instance a stator core of the motor may have two air gaps of differing gap widths on opposing sides of the stator for preventing magnetic saturation therein. As a rotor of the machine rotates within the surrounding stator, the magnetic poles of the rotor create asymmetric reluctance paths about the stator core in accordance with the angular position of the rotor's magnetic poles and the gap width ratio between the two air gaps of the stator.
According to the alternative "homopolar flux effects" mechanism outlined by Kerszenbaum, the induced currents are attributed to a form of axial magnetization. However, the homopolar flux effects mechanism is considered primarily relevant to only large and fast rotating machines, i.e., turbo generators, and is of little consequence for most machines driven by solid-state, variable-speed drive.
J. Allan Lawson, "Motion Bearing Fluting", 1993 Annual Pulp and Paper industries Technical Conference, 1993 Conference Record, pages 3214 35 discusses dynamic changes inside an electric motor as causing induced shaft currents, with non-uniform air-gap magnetic flux transitions within the motor being the primary cause. An air-gap flux of an electric motor originates from "field" (stator) poles of the motor. Ideally each field pole is identical to the other field poles for providing balanced excitation of the motor. However, in reality, the windings of the various poles differ from one another resulting in non-uniform excitation of the motor such that non-uniform magnetic flux transients cross the shaft for inducing currents within the shaft. The magnitude of the induced currents is determined in accordance with the extent of the magnetic non-uniformity and the rate of change associated with the flux transitions.
Solid state variable speed drives are used with electrical motors for controlling the speed of the motor. For DC motors, pulse width or phase duty cycle modulation is employed for adjusting the average DC current level. For AC synchronous motors, the solid state switches provide pulse trains for synthesizing a desired frequency signal. However, the switching transients associated with these techniques produce abrupt current transients causing rapid magnetic flux transitions within the motor. Thus, a motor employing a solid state variable speed drive, and having a given level of magnetic imbalance, will suffer induced current levels of a greater magnitude than the same motor without the variable speed drive.
The relationship can be explained in accordance with Lenz's law wherein, ##EQU1## Voltage is generated by the changing magnetic field wherein the voltage magnitude is related to the rate of flux variation. Thus, for the induced currents in a motor shaft, the solid-state devices of a variable-speed controller, which rapidly switch currents through the field or armature windings generate magnetic fields which change quickly with respect to time. If the flux lines cross the shaft in a non-uniform manner, the flux transitions induce voltages, and likewise currents, within the shaft at levels related to the rate of change of the flux transition and the motor's magnetic imbalance.
As with the shaft of an electrical motor, it is also possible for a non-driven shaft to electrostatically accumulate charge, acquiring a voltage level. For instance, paper generating static electricity while being rolled about the shaft of a roller can cause charge to accumulate on the shaft until a given voltage level is built up whereupon an electrical discharge occurs through a bearing/race assembly journalling the shaft, causing pitting.
Regardless of the manner in which the shaft acquires a voltage or current, electrical discharge through the bearing assembly is undesirable in that such electrical discharge leads to bearing failure. The electrical discharge pits the bearing and race at the discharge interface. Eventually the pitting leads to "fluting" (a characteristic wear within the assembly) and failure. To prevent premature failure, the motor may be constructed magnetically symmetric, or an alternative shaft discharge path provided around the bearing assembly. One known discharge apparatus for a rotating shaft which is typically steel, comprises a grounding brush contacting the outside diameter of the rotating shaft. However, the electrical interface of a steel motor shaft and carbon brush is less than ideal for providing a reliable electrical contact therebetween. The steel shaft of the motor builds up an insulating oxide layer for degrading the electrical interface, while grease and oil from the motor or roller bearing assembly accumulate on the shaft and contaminate the electrical interface. Therefore, in order to assure the reliability for such a grounding brush technique, it would be necessary that the brush/shaft assembly be cleaned or maintained frequently to prevent oxidation, film and contamination build-up for assuring reliable electrical contact therebetween.
Another prior art technique employs a copper ring within a motor which is received onto the shaft in a heated condition and then allowed to cool for providing a snug fit about the shaft, providing a copper contacting surface for a grounding brush. However, the copper can corrode and likewise accumulate grease and external matter thereon for the same reasons that the shaft does. Furthermore, to fit a heated copper ring over the shaft, it is required that the motor be disassembled for inserting the copper ring. Such a disassembly can be even more burdensome than continually cleaning the shaft.
Therefore, it is an object of the present invention to provide an improved apparatus for discharging a rotating shaft.
It is another object of the present invention to provide the discharge apparatus with a reliable low-impedance discharge path.
It is another object of the present invention to provide an apparatus for discharging a rotating shaft which apparatus may be easily assembled to an existing motor.
Finally, it is desirable the discharging apparatus be resistant to environmental factors.